Question

Identify the complement activation pathway that is triggered by the binding of an acute-phase protein to a pathogen. a. classical b. alternate c. lectin d. cathelicidin

Short answer

c. lectin

Step-by-step solution

- Understand the role of acute-phase proteins

Acute-phase proteins are a class of proteins whose plasma concentrations increase or decrease in response to inflammation. One of their key functions is to bind to pathogens, leading to the activation of the complement system.

- Review the complement activation pathways

There are three main complement activation pathways: classical, alternate, and lectin. Each pathway is triggered by different mechanisms: the classical pathway is typically activated by antibodies, the alternate pathway by spontaneous hydrolysis or pathogen surfaces, and the lectin pathway by mannose-binding lectin (MBL) binding to pathogen surfaces.

- Identify the binding specifics of the acute-phase protein

Mannose-binding lectin (MBL) is an example of an acute-phase protein that binds specific carbohydrate patterns on the surface of pathogens.

- Determine the pathway activated by MBL

Since MBL is an acute-phase protein that binds to pathogen surfaces and triggers the activation of the complement system, it activates the lectin pathway.

- Select the correct answer

Based on the analysis, the lectin pathway is activated by the binding of an acute-phase protein to a pathogen.

Key concepts

Acute-Phase Proteins

Acute-phase proteins are essential components of our immune system. Their levels in the blood change during inflammation. These proteins have several important functions:

• They help in trapping pathogens, making it easier for immune cells to capture and destroy them.
• They also play a role in activating the complement system, a part of our immune defense.

One well-known acute-phase protein is mannose-binding lectin (MBL). When MBL binds to the surface of a pathogen, it triggers a cascade of immune responses. This helps the body quickly respond to potential threats.

Classical Pathway

The classical pathway is one of the three pathways that activate the complement system. It is typically triggered by:

• The binding of antibodies to antigens (foreign substances).
• Immune complexes (antigens bound to antibodies).

When antibodies recognize and attach to a pathogen surface, they can activate the classical pathway. This leads to the formation of C1 complex, which then triggers a series of reactions resulting in the destruction of the pathogen. The key components of the classical pathway are: C1, C2, C3, and C4, which work together to tag and eliminate the pathogens.

Alternate Pathway

The alternate pathway provides a rapid response to infections. It does not rely on antibodies or MBL. Instead, it is activated by:

• Spontaneous hydrolysis of C3 in the blood.
• Direct interaction with pathogen surfaces.

This pathway continuously surveys for pathogens, ready to initiate an immune response. Once activated, it leads to the formation of C3 convertase, which plays a crucial role in opsonization (marking pathogens for destruction) and the formation of the membrane attack complex (MAC). The alternate pathway is vital for early defense against infections, especially before the adaptive immune response kicks in.

Lectin Pathway

The lectin pathway is initiated when mannose-binding lectin (MBL) attaches to carbohydrates on the surface of pathogens. Components of the lectin pathway include:

• MBL (an acute-phase protein).
• MASP-1 and MASP-2 (MBL-associated serine proteases).

Once MBL binds to the pathogen, MASP-1 and MASP-2 become activated. This leads to the cleavage and activation of complement proteins C4 and C2, forming C3 convertase. The lectin pathway shares many downstream components with the classical pathway, resulting in similar immune responses. This pathway is crucial for recognizing and responding to pathogens that display specific carbohydrate patterns on their surfaces.